The Applicant has developed a range of Memjet® inkjet printers as described in, for example, WO2011/143700, WO2011/143699 and WO2009/089567, the contents of which are herein incorporated by reference. Memjet® printers employ a stationary pagewidth printhead in combination with a feed mechanism which feeds print media past the printhead in a single pass. Memjet® printers therefore provide much higher printing speeds than conventional scanning inkjet printers.
An inkjet printhead is comprised of a plurality (typically thousands) of individual inkjet nozzle devices, each supplied with ink. Each inkjet nozzle device typically comprises a nozzle chamber having a nozzle aperture and an actuator for ejecting ink through the nozzle aperture. The design space for inkjet nozzle devices is vast and a plethora of different nozzle devices have been described in the patent literature, including different types of actuators and different device configurations.
One of the most important criteria in designing an inkjet nozzle device is achieving ink drop trajectories perpendicular to the nozzle plane. If each drop is ejected perpendicularly outward, the tail following the drop will not catch and deposit on the nozzle edge. A source of flooding and drop misdirection is thus avoided. Additionally, with perpendicular trajectories, the primary satellite formed by breakup of the drop tail can be made to land on top of the main drop on the page, hiding that satellite. Significant improvements in print quality can thus be obtained with perpendicular drop trajectories.
Memjet® inkjet printers are thermal devices, comprising heater elements which superheat ink to generate vapor bubbles. The expansion of these bubbles forces ink drops through the nozzle apertures. To ensure perpendicular trajectories for these drops, the bubbles must expand symmetrically. This requires symmetry in the design of the nozzle device.
Perfect fluidic symmetry around the heater element is not possible unless the heater element is suspended directly over the inlet to the nozzle chamber. Inkjet nozzle devices having this arrangement are described in, for example, U.S. Pat. No. 6,755,509, and a printhead comprising such a device is shown in U.S. Pat. No. 7,441,865 (see, for example, FIG. 21B), the contents of which are herein incorporated by reference. However, devices having a heater element suspended over the chamber inlet require relatively complex fabrication methods and are less robust than devices having bonded heater elements. Furthermore, these devices suffer from a relatively high rate of backflow through the chamber inlet during ink ejection (resulting in inefficiencies), as well as potential printhead face flooding during chamber refilling by virtue of the alignment of the inlet and the nozzle aperture.
U.S. Pat. No. 7,857,428 describes an inkjet printhead comprising a row of nozzle chambers, each nozzle chamber having a sidewall entrance which is supplied with ink from a common ink supply channel extending parallel with the row of nozzle chambers. The ink supply channel is supplied with ink via a plurality of inlets defined in a floor of the channel. The entrance to each nozzle chamber may comprise a filter structure (e.g. a pillar) for filtering air bubbles or particulates entrained in the ink. The arrangement described in U.S. Pat. No. 7,857,428 provides redundancy in the supply of ink to the nozzle chambers, because all nozzle chambers in the same row (or pair of rows) are supplied with ink from the common ink supply channel extending parallel therewith. However, the arrangement described in U.S. Pat. No. 7,857,428 suffers from the disadvantages of relatively slow chamber refill rates and fluidic crosstalk between nearby nozzle chambers.
In addition, the arrangement described in U.S. Pat. No. 7,857,428 inevitably introduces a degree of asymmetry into droplet ejection compared to the arrangement described in U.S. Pat. No. 6,755,509. Since the heater element is laterally bounded by the chamber sidewalls except for the chamber entrance, the bubble generated by the heater element is distorted by this asymmetry. In other words, some of the impulse generated by the bubble tends to force some ink back through the chamber entrance as well as through the nozzle aperture. This results in skewed droplet ejection trajectories as well as a reduction in efficiency.
One measure for addressing the asymmetry caused by a sidewall chamber entrance is to lengthen and/or narrow the chamber entrance to increase its fluidic resistance to backflow. However, this measure is not viable in high-speed printers, because it inevitably reduces chamber refill rates due to the increased flow resistance. An alternative measure which compensates for the asymmetry caused by a sidewall chamber entrance is to offset the heater element from the nozzle aperture, as described in U.S. Pat. No. 7,780,271 (the contents of which is incorporated herein by reference).
It would be desirable to provide an inkjet nozzle device, which has a high degree of symmetry so as to minimize the extent of any compensatory measures required for correcting droplet ejection trajectories. It would further be desirable to provide an inkjet nozzle device having a high chamber refill rate, which is suitable for use in high-speed printing. It would further be desirable to provide an inkjet printhead having minimal fluidic crosstalk between nearby nozzle devices.
Furthermore, the high density of nozzle devices in a typical pagewidth printhead poses a thermal management problem: the ejection energy per drop ejected must be low enough to operate in so-called ‘self-cooling’ mode—that is, the chip temperature equilibrates to a steady state temperature well below the boiling point of the ink via removal of heat by ejected ink droplets.
Conventional inkjet nozzle devices comprise resistive heater elements coated with a number of relatively thick protective layers. These protective layers are necessary to protect the heater element from the harsh environment inside inkjet nozzle chambers. Typically, heater elements are coated with a passivation layer (e.g. silicon dioxide) to protect the heater element from corrosion and a cavitation layer (e.g. tantalum) to protect the heater element from mechanical cavitation forces experienced when a bubble collapses onto the heater element. U.S. Pat. No. 6,739,619 describes a conventional inkjet nozzle device having passivation and cavitation layers.
However, multiple passivation and cavitation layers are incompatible with low-energy ‘self-cooling’ inkjet nozzle devices. The relatively thick protective layers absorb too much energy and require drive energies which are too high for efficient self-cooling operation.
U.S. Pat. No. 6,113,221 describes an inkjet nozzle device, which vents gas bubbles through nozzle apertures during droplet ejection. By venting gas bubbles, instead of the gas bubbles collapsing onto the heater element, the damaging effects of cavitation forces can be avoided. Consequently, heater elements without cavitation layer(s) may be employed, which improves thermal efficiency. However, the inkjet nozzle devices described in U.S. Pat. No. 6,113,221 are configured to evacuate the entire nozzle chamber of ink during droplet ejection such that the volume of ejected droplets is substantially equal to the volume of the nozzle chamber. This places constraints on nozzle chamber designs for a target drop ejection volume.
It would be desirable to provide inkjet nozzle devices which vent gas bubbles, whilst allowing more flexible design criteria than the venting devices described in the prior art.